In an audio signal processing system, elements within the signal processing paths, such as audio amplifiers, or signal converters, will often display nonlinear characteristics. Because of this, there is a possibility that received signals at frequencies outside the audio band will be downconverted or mixed down to frequencies within the audio band. In some situations, this can impact on downstream system performance of speech recognition systems or speaker recognition systems. These out-of-band signals may be generated innocently, for example in ultrasonic transmitters used as distance sensors, or in ultrasonic communication systems, or even when metal objects such as coins or keys are brought into contact with each other. However, it is also possible that out-of-band signals may be generated deliberately in an attempt to maliciously affect system performance, for example as part of an injection attack or a denial-of-service attack.